The figures are breathtaking. Le Monde is publishing the first part of an investigation that is spectacular and unprecedented in size. A team of journalists in Paris, Washington, Geneva and 46 other countries has uncovered the hidden workings of a giant tax evasion scheme conducted with the knowledge, and indeed the encouragement, of the world’s second largest bank, HSBC, via its Swiss subsidiary, HSBC Private Bank.

Le Monde has been investigating the HSBC affair since the outset, and in early 2014 came into possession of international banking data for the period 2005-07 that provided evidence of fraud on a gigantic scale. We shared this information with some 60 other news outlets around the world, coordinated by the ICIJ, the International Consortium of Investigative Journalists. The publication of these figures is likely to prove embarrassing for a good number of celebrities, including the French comedian Gad Elmaleh and the King Mohamed VI of Morocco, but it will shake the international financial industry to the core.

The investigators allege that between 9 November 2006 and 31 March 2007 180.6 billion euros passed through accounts held by over 100,000 clients and 20,000 offshore companies with HSBC in Geneva. This is the period covered by digital files stolen from HSBC Private Bank by one of its former staffers, Hervé Falciani.

In late 2008 this French computer analyst passed on information he had stolen from his employer to the French tax authorities. The case was referred to French prosecutors in January 2009, and they have since concentrated their investigations on a small part of the “Falciani lists” – the 3,000 or so French citizens suspected of having concealed their money at HSBC PB with the bank’s collusion. The bank has de facto been placed under formal investigation as a legal entity for “illicit selling of banking and financial services” and “money laundering of the proceeds of tax evasion”.

HSBC PB is said to have concealed over €5.7 billion in tax havens for its French clients alone. The French Finance Ministry is taking only 72 individuals to court (including Nina Ricci’s heir, whose trial is to begin in Paris in the coming days), as most of the French taxpayers whose identity was revealed in the Falciani lists have since regularised their tax affairs.

A few days after those articles were published, a person appeared at Le Monde’s reception on Boulevard Auguste Blanqui in Paris. This source, whose identity shall remain secret, gave us a memory stick containing all the files the French tax authorities had extracted from the Falciani lists in the strictest secrecy since 2009, sometimes ignoring government reservations.

Who features on these lists that the French Treasury sent to several foreign administrations, and whose names we shall publish if we consider it to be in the public interest? Arms traffickers and drug smugglers, funders of terrorist organisations, politicians, celebrities, sporting idols and captains of industry, most of them keen to hide their money in Switzerland. It was perfectly illegal for the French clients and for many others to do this. The holders of the accounts are striking in their variety: French surgeons wishing to launder undeclared fees rub shoulders with the protagonists of the Elf affair, Belgian diamond traders, Al-Qaeda funders and a large number of Jewish families who had sent their assets to Switzerland for safekeeping as the Nazis took control of Europe.

Offshore structures as screens

The bank’s account managers illegally contacted many of these people in France. They were all encouraged by HSBC PB’s executive committee to hide their money behind offshore screen structures, generally based in Panama or the British Virgin Islands, in order to avoid certain European taxes, chiefly the EU Savings Directive, a withholding tax introduced in 2005. Investigators now have material evidence to prove these various crimes.

An exceptional affair calls for exceptional measures so, as the exclusive recipient of this highly sensitive information, Le Monde decided in spring 2014 to share it with the international press to ensure that it would be treated as completely and accurately as possible. We therefore chose to send it to the US-based ICIJ, our partner for “Offshore Leaks” (in 2013) and “LuxLeaks” (in 2014). In total secrecy, the ICIJ mobilised 154 journalists from 47 media outlets in various countries (the Guardian in Great Britain, the Süddeutsche Zeitung in Germany, CBS’s 60 Minutes in the United States).

On a Monday in September 2014, 40 journalists from around the world gathered in a room at Le Monde’s offices. Some of them had only received a few days’ notice. They knew that this was a major story, but the mass of confidential information they were about to discover was beyond their wildest imaginings.

The meeting lasted nine hours, with only short breaks to eat. It marked the launch of one of the largest journalistic operations this century. Given the sheer quantity of information, Le Monde had quickly realised that it wouldn’t be able deal with the documents on its own. The decision to share access to the data with the International Consortium of Investigative Journalists (ICIJ) is symbolic of a new era in international investigative journalism - let’s call it “2.0” for short. The ICIJ in turn set up a network of unprecedented size to handle the HSBC data.

At that time, the operation hadn’t yet become “SwissLeaks” but was known as “Voyager”, a codename taken from the TV series Star Trek, as is the ICIJ’s custom. That Monday in September the meeting’s participants learned about the three tools that the consortium had put in place at great speed over the summer. The first was a secure forum, a kind of closed Facebook on which to post their discoveries; the second, “Blacklight”, a database that was searchable by name, country and source; and the third, “Graph”, an app to visualise the links between assignees and offshore holdings and companies. All communications would be encrypted.

Then everyone set to work, each in their own way. At the Guardian, four journalists papered the walls of a room with hundreds of Post-Its, arranged by category: politicians, celebrities, drug dealers, philanthropists, arms dealers, and so on. Le Monde also set up a special unit. One thing they still had to decide upon was the date and time of publication. There was some tough negotiating due to the number of different time zones involved and the timing of programmes such as CBS’s 60 Minutes show, which goes out every Sunday evening at 7.30 p.m., New York time. What’s more, the journalists had to call up every single person mentioned in the articles, whether that person was in Paris or Bamako, in the final feverish weeks before the fatal deadline: Sunday 8 February at 10 p.m. CET.

HSBC Private Bank and the Swiss political and fiscal authorities have, since the very beginning, disputed both the figures established by the tax office and prosecutors in France and the use of this data, arguing that it was stolen. The thief, Hervé Falciani, first tried to sell the files before changing his mind and turning them over to the French authorities. He was charged by the Office of the Attorney General of Switzerland on 11 December 2014 for “industrial espionage”, “data theft” and “breach of industrial and banking secrecy”.

Switzerland looks unfavourably on the investigations being conducted by the French judicial and tax authorities, and insists that the original data has been tampered with, something French investigators categorically deny. On 27 February 2014 the two investigating magistrates concluded with regard to the lists that their “authenticity [had] been verified in hearings with numerous account-holders, who have, what is more, come to an agreement with the tax authorities on the basis of this file”. For its part, HSBC PB seems prepared to follow suit with the French judicial authorities to avoid a trial that could be ruinous for the bank – and not just in terms of its reputation.